Reactive Thrombocytosis in Febrile Young Infants with Serious Bacterial Infection

Objective To estimate the incidence of reactive thrombocytosis among febrile young infants and to asses the utility of platelet count as a potential predictor of serious bacterial infection (SBI) Design Retrospective study between January 2005 and December 2008 Setting Tertiary care pediatric unit Participants All infants 29 to 89 days of age admitted with rectal temperature >38 degrees C without a focus of infection Main Outcome Measures The results of the sepsis evaluation on admission were recorded SBI included all cases of occult bacteremia urinary tract infection bacterial meningitis pneumonia bacterial gastroenteritis and infections of the soft tissues and bones Results Of the 408 infants studied 103 (25 2%) had SBI Platelet count was significantly higher in infants with SBI compared to those without (median 513000 /mm(3) [interquartile range 455 000-598 000/mm(3)] vs median 398000/mm(3) [interquartile range 313 000-463 000/mm(3)] P<0 001) Thrombocytosis had only moderate ability in predicting SBI (area under the curve 0 74 95%Cl 0 70-0 79) The combination of platelet count >= 450 000/mm(3) WBC >= 15 000/mm(3) C-reactive protein >= 2 mg/dL and pyuria >= 10 WBC/hpf would lead to misclassification of 4 infants with SBI (3 9% of SBIs negative likelihood ratio 0 08) Conclusions Reactive thrombocytosis was a frequent finding in young infants with SBI Thrombocytosis >= 450 000 cells/mm(3) in combination with leucocytosis elevated CRP and pyuria, may help in early recognition of febrile young infants at risk for SBI